Process for preparing aqueous polymer dispersions

ABSTRACT

Redispersible polymer powders are prepared by emulsion polymerization of ethylenical, unsaturated monomers in the presence of a protective colloidal stabilizer containing 20 to 95 weight percent of polymerized monomers containing sulfonic acid or sulfonate groups, 5 to 80 weight percent of polymerized monomers containing amide groups or hydroxylalkyl groups, and up to 5 weight percent of polymerized monomers which are hydrophobic, water-insoluble ethylenically unsaturated compounds. The redispersible polymers are suitable for use in construction materials and coatings, and exhibit higher water resistance than similar polymers stabilized with polyvinyl alcohol as a colloidal stabilizer.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The invention relates to a process for preparing aqueous polymerdispersions, and also to their use for preparing water-redispersiblepolymer powders.

2) Background Art

Protective-colloid-stabilized aqueous polymer dispersions, for examplefor construction sector applications, are known. To ensure that theaqueous polymer dispersions are stable, protective colloids, such aspolyvinyl alcohol, polyvinylpyrrolidone, cellulose or starch, are usedduring the polymerization in an aqueous medium. EP-A 133899 (U.S. Pat.No. 4,532,295) has disclosed starch-stabilized polymer dispersions. WO-A84/00369 (U.S. Pat. No. 4,528,315) and EP-B 62106 (U.S. Pat. No.4,397,968) describe dispersions stabilized using polyvinyl alcohol.

However, the addition of significant amounts of protective colloids,which are necessary to ensure sufficient stability, is attended bydisadvantages: protective colloids are relatively expensive, theiraddition reduces the binder content of the dispersion, rheologicaldisadvantages result and, finally, the addition of protective colloidsmakes the polymer films produced from the dispersion susceptible toattack by water. The susceptibility to attack by water is, inparticular, undesirable in many applications in the construction sectorand coatings sector, and has to be counteracted by subsequent stepsduring formulation. Reactivity with water is particularlydisadvantageous in highly polymer-filled cement applications, such assealing slurries.

To improve the water resistance of polymer films which have beenprepared using protective-colloid-stabilized polymer dispersions EP-A727441 recommends the use of water-soluble protective colloids based onfrom 5 to 50% of comonomers with acid and/or anhydride functions, from0.1 to 80% of long-chain (meth)acrylates and up to 94.9% ofwater-insoluble principle monomers, such as (meth)acrylates or vinylesters. A disadvantage is that these protective colloids are prepared bypolymerization in organic solvents, and complicated solvent removal bydistillation is required.

DE-A 2618898 (U.S. Pat. No. 3,965,032) describes protective colloidsbased on copolymer polyelectrolytes having nonionic hydrophobic units,such as methyl methacrylate, and hydrophilic units, such asethylenically unsaturated comonomers substituted with sulfonate groupsand prepared by polymerization in organic solvents. These protectivecolloids have disadvantages which are the same as the abovementionedEP-A 727441.

DE-A 19608911, which is a subsequent publication, disclosescrosslinkable protective colloids having sulfonate-containing,N-methylol-containing and hydrophobic monomer units. A disadvantage isthat when polymers stabilized with protective colloids of this type areused in acid media or exposed to heat they crosslink and thereforebecome brittle. This is contrary to the properties of flexibilityrequired in many applications (sealing slurries, sealing compositions,and paints for bridging cracks).

EP-B 206814 describes the use of terpolymers made from (meth)acrylicacid and from sulfonate-functional monomer and vinyl esters forstabilizing aqueous systems which comprise dissolved or suspendedsolids, for example cooling water or boiler water.

EP-A 671435 discloses that copolymers made from alkyl acrylates and fromsulfonate- or carboxylate-substituted monomers are suitable sprayingaids for spraying protective-colloid-stabilized dispersions. EP-A 629650(U.S. Pat. No. 5,462,978) describes copolymers made fromsulfonic-acid-functional comonomers and from water-insoluble comonomersas spraying aids in the spray drying of aqueous polymer dispersions.

SUMMARY OF THE INVENTION

The object on which the invention was based was to provideprotective-colloid-stabilized aqueous polymer dispersions which havebetter water resistance when used in the construction sector or coatingssector than previously known polymer dispersions, for example thosestabilized with polyvinyl alcohol.

The invention provides a process for preparingprotective-colloid-stabilized aqueous polymer dispersions byfree-radical polymerization of ethylenically unsaturated monomers byemulsion polymerization, which comprises carrying out the polymerizationin the presence of one or more protective colloids comprising

a) from 5 to 95% by weight of monomer units containing sulfonic acidgroups or sulfonate groups,

b) from 5 to 95% by weight of noncrosslinkable, water-soluble monomerunits, and

c) from 0 to 5% by weight of hydrophobic monomer units selected from theclass consisting of water-insoluble ethylenically unsaturated compounds,where the proportions in % by weight are based on the total weight ofthe copolymer.

DESCRIPTION OF THE REFERRED EMBODIMENTS

Suitable monomer units a) are water-soluble, free-radical-polymerizable,ethylenically unsaturated compounds which contain sulfonic acid groupsand, respectively, sulfonate groups —SO₃M, where M═H, an alkali metalion, an ammonium ion or an alkaline earth metal ion. Preference is givento 2-acrylamido-2-methylpropanesulfonic acid (AMPS), styrenesulfonicacid, sulfoalkyl (meth)acrylates, sulfoalkyl itaconates, in each casepreferably having a C₁-C₆-alkyl radical, vinylsulfonic acid andammonium, alkali metal or alkaline earth metal salts thereof. Particularpreference is given to 2-acrylamido-2-methylpropane-sulfonic acid(AMPS), styrenesulfonic acid, sulfopropyl acrylate, sulfopropylitaconate, vinylsulfonic acid and ammonium, sodium, potassium andcalcium salts thereof.

Preferred monomer units b) are water-soluble,free-radical-polymerizable, ethylenically unsaturated compounds whichcontain carboxyl groups —COOM, where M═H, an alkali metal ion, ammoniumion or alkaline earth metal ion, or contain amide groups —CONH₂ orcontain hydroxyl groups. Particular preference is given to acrylic acid,methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleicacid, hydroxyalkyl (meth)acrylates, such as hydroxyethyl acrylate orhydroxypropyl acrylate, hydroxybutyl acrylate, acrylamide andmethacrylamide. Acrylamide and methacrylamide are most preferred.

For the purposes of the present invention, water-soluble generallyimplies solubility in water of at least 10% by weight at 23° C.

Suitable monomer units c) are free-radical-(co)polymerizable,ethylenically unsaturated compounds which have less than 4% by weightwater-solubility at 23° C. Preference is given to esters of acrylic ormethacrylic acid with alcohols having from 1 to 18 carbon atoms, such asmethyl methacrylate, methyl acrylate, N-butyl acrylate, 2-ethylhexylacrylate; vinylaromatics, such as styrene or vinyltoluene; olefins, suchas ethylene, propylene or butadiene; vinyl halides, such as vinylchloride; vinyl esters of aliphatic carboxylic acids having from 1 to 18carbon atoms, such as vinyl acetate, vinyl propionate, isopropenylacetate, vinyl laurate, and vinyl esters of α-branched monocarboxylicacids having from 5 to 11 carbon atoms, such as VeoVa9® or VeoVa10®,Shell products which are vinyl esters of verstatic acid, or a saturatedmonocarboxylic acid mixture of highly branched C-9 and C-10 isomers.

Preferred protective colloids have

a) from 20 to 95% by weight of monomer units containing sulfonic acidgroups and/or sulfonate groups and

b) from 5 to 80% by weight of monomer units containing carboxyl groups,amide groups or hydroxyalkyl groups, in particular protective colloidshaving

a) from 20 to 95% by weight of monomer units containing sulfonic acidgroups or sulfonate groups and

b) from 5 to 80% by weight of monomer units containing amide groups,where the proportions in % by weight are in each case based on the totalweight of the copolymer.

Particularly preferred protective colloids have

a) from 40 to 60% by weight of monomer units which derive from one ormore monomers selected from the class consisting of2-acrylamido-2-methylpropane-sulfonic acid, styrenesulfonic acid,sulfoalkyl (meth)acrylates, sulfoalkyl itaconates, in each casepreferably with a C₁-C₆-alkyl radical, vinylsulfonic acid and saltsthereof, and

b) from 40 to 60% by weight of monomer units which derive fromacrylamide and/or from methacrylamide.

The protective colloids are preferably prepared by free-radicalpolymerization in aqueous solution at a reaction temperature ofpreferably from 40 to 80° C. Initiation is by the usual water-solublefree-radical generators, preferably used in amounts of from 0.01 to 3.0%by weight, based on the total weight of the monomers. Examples of theseare ammonium persulfate, potassium persulfate, hydrogen peroxide,potassium peroxodiphosphate, sodium peroxodiphosphate and ammoniumperoxodiphosphate. If desired, the free-radical initiators mentioned mayalso, in a known manner, be combined with from 0.01 to 1.0% by weight,based on the total weight of the monomers, of reducing agents. Examplesof suitable reducing agents are alkali metal formaldehyde sulfoxylatesand ascorbic acid. To adjust the molecular weight, the regulatorsusually used may be added during the polymerization, for examplemercaptans, aldehydes and chlorinated hydrocarbons. The copolymers arepreferably used in the form of their aqueous solutions. Solids contentsare preferably adjusted to from 15 to 25% by weight, depending on theapplications.

The novel process is suitable for free-radical polymerization ofethylenically unsaturated monomers, for example of vinyl esters ofunbranched or branched carboxylic acids having from 1 to 18 carbonatoms, of esters of acrylic or methacrylic acid with unbranched orbranched alcohols having from 1 to 18 carbon atoms, of vinylaromatics,of vinyl halides and of olefins.

Preferred vinyl esters are vinyl acetate, vinyl propionate, vinylbutyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate,vinyl pivalate and vinyl esters of α-branched monocarboxylic acidshaving 5 or 9 to 11 carbon atoms, for example VV5^(R), VeoVa9^(R) orVeoVa10^(R). Vinyl acetate is particularly preferred.

Preferred methacrylates or acrylates are methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate,propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutylacrylate, isobutyl methacrylate, tert-butyl acrylate, tert-butylmethacrylate, 2-ethylhexyl acrylate and 2-ethylhexyl methacrylate.Particular preference is given to methyl acrylate, methyl methacrylate,n-butyl acrylate and 2-ethylhexyl acrylate.

Preferred vinylaromatics are styrene, α-methylstyrene, o-chlorostyreneor vinyltoluenes. Preferred vinyl halides are vinyl chloride andvinylidene chloride. Preferred olefins are ethylene, propylene,1,3-butadiene and isoprene.

If desired, from 0.05 to 30.0% by weight, preferably from 0.5 to 15% byweight, based in each case on the total weight of the monomers, of oneor more comonomers may also be added, for example for crosslinking or tomodify the adhesion properties of the polymers prepared according to theinvention. The amounts of crosslinking monomers used are preferably from0.5 to 5.0% by weight, based on the total weight of the monomers.Examples of these are N-methylolacrylamide, N-(alkoxymethyl)acrylamidesor N-(alkoxymethyl)methacrylamides having a C₁-C₆-alkyl radical, such asN-(isobutoxymethyl)acrylamide (IBMA), N-(n-butoxymethyl)acrylamide(NBMA); comonomers with more than one ethylenic unsaturation, such asethylene glycol diacrylate, divinyl adipate, divinylbenzene, diallylphthalate or triallyl cyanurate. Examples of comonomer units suitablefor modifying adhesion properties are hydroxyalkyl methacrylates, andhydroxyalkyl acrylates, such as hydroxyethyl, hydroxylpropyl orhydroxylbutyl acrylate or methacrylate, and also compounds such asdiacetoneacrylamide and acetylacetoxyethyl acrylate or methacrylate.

Using the novel method, the aqueous polymer dispersions are prepared byaqueous emulsion polymerization in the presence of from 1 to 20% byweight, preferably from 5 to 10% by weight, of one or more of theprotective colloids mentioned, based in each case on the total weight ofthe monomers.

The polymerization temperature is generally from 35 to 95° C.,preferably from 40 to 80° C. The polymerization may be carried out as abatch process, where all of the components are in the initial reactorcharge, or by the feed process, where one or more components are addedduring the polymerization. Variants of mixing which have an initialcharge and a feed are preferred. The feeds may be separate (spatiallyand chronologically) or some or all of the components to be fed may bepreemulsified before being fed.

The protective colloids may be within the initial charge or in the feed,or divided between initial charge and feed. The protective colloids arepreferably within the initial charge in the form of their aqueoussolutions.

If, for example, gaseous reaction components are used, the emulsionpolymerization may also be carried out at elevated pressure. Foroperations under elevated pressure, for example when using the monomersvinyl chloride or ethylene, pressures of from 5 bar to 100 bar arepreferred. An example of a factor in the decision is the desired amountof ethylene to be polymerized.

The protective colloids toube used according to the invention may beused either alone or combined with emulsifiers. Accompanying emulsifiersmay be anionic or nonionic emulsifiers. If the polymerization is carriedout in the presence of emulsifiers, the amount of these is preferably upto 4% by weight, based on the total weight of the monomer phase.Preference is given to the use of anionic or nonionic emulsifiers.Examples of commonly used emulsifiers are ethoxylated fatty alcoholshaving a C₈-C₃₆-alkyl radical and an ethoxylation number (EO number) offrom 3 to 50; ethoxylated mono-, di- and trialkylphenols having aC₄-C₁₀-alkyl radical and an EO number of from 3 to 50; alkali metalsalts of di-C₄-C₁₂-alkyl esters of sulfosuccinic acid. Other suitablecompounds are alkali metal salts and ammonium salts of C₈-C₁₂-alkylsulfates, of ethoxylated alkanols having a C₁₂-C₁₈-alkyl radical and anEO number of from 3 to 30, of ethoxylated C₄-C₁₀-alkylphenols having anEO number of from 3 to 50, of C₁₂-C₁₈-alkylsulfonic acids, ofC₉-C₁₈-alkylarylsulfonic acids and of sulfonates of ethoxylated, linearor branched C₈-C₃₆-alkylalcohols having an EO number of from 3 to 50. Inthe most preferred embodiment the polymerization is carried out withoutadding emulsifier.

The polymerization is initiated with the water-soluble, thermalinitiators or redox-initiator combinations commonly used for emulsionpolymerization. Examples of thermal initiators are organic peroxides,such as tert-butyl hydroperoxide and cumyl hydroperoxide, orpersulfates, such as potassium persulfate, or H₂O₂, or azo compounds,such as azodiisobutyronitrile. Redox initiators preferably used arehydrogen peroxide, tert-butyl hydroperoxide or potassium persulfatecombined with hydroxymethanesulfinic acid, ascorbic acid or sodiumsulfite as reducing agent. The amount of initiator is preferably from0.01 to 1.0% by weight, based on the total weight of the monomer phase.

To control the molecular weight, regulating substances may be usedduring the polymerization. The amounts of these used are usually from0.01 to 5.0% by weight, based on the monomers to be polymerized, andthey are fed separately or else premixed with reaction components.Examples of substances of this type are dodecylmercaptan,mercaptopropionic acid, methyl mercaptopropionate, isopropanol andacetaldehyde.

The aqueous dispersions obtainable by the novel process have a solidscontent of from 30 to 75% by weight, preferably from 40 to 65% byweight.

The protective-colloid-stabilized, aqueous polymer dispersions are alsosuitable for producing water-redispersible polymer powders. For this,the aqueous dispersions are dried. The dispersions are preferablyspray-dried or freeze-dried. The dispersions are most preferablyspray-dried.

As spraying aids, to ensure redispersibility it is necessary to addfurther water-soluble protective colloids to the dispersion prior todrying. These are different from the sulfonic-acid-functional or,respectively, sulfonate-functional protective colloids used for thepolymerization. Examples of substances of this type which are widelycommercially available are: polyvinyl alcohols, polyvinylpyrrolidone,cellulose derivatives, starch derivatives, and water-solublecondensation products made from melamine and formaldehyde or fromnaphthalenesulfonic acid and formaldehyde. Preference is given topolyvinyl alcohols. The amount of the spraying aid generally used isfrom 5 to 25% by weight, based on the polymeric constituents of thedispersion.

A content of up to 1.5% by weight of antifoam, based on the basepolymer, has frequently proven advantageous during spraying. To increasestorage capability by improving resistance to blocking, in particularfor powders with a low glass transition temperature, the powder obtainedcan be admixed with an antiblocking agent (anticaking agent), preferablyup to 30% by weight, based on the total weight of polymericconstituents.

Surprisingly, and despite their redispersibility, the dispersion powdersobtained in this way give, for example in modifying hydraulicallysetting compositions of building materials, products with better waterresistance than is given by conventional dispersion powders.

The protective-colloid-stabilized polymer dispersions and the dispersionpowders obtainable therefrom are suitable as binders for coatings andrenders, in particular paints; as adhesives or binders for wood, paper,textiles or nonwovens; as binders in papermaking and for producingmolding compositions and moldings; as binders for use in theconstruction industry, in particular as additives to concrete, toconstruction adhesives, to mortars, to troweling compositions and toleveling compositions.

The examples below further illustrate the invention.

Preparation of the Protective Colloids

EXAMPLE 1

2600 g of water, 40 g of 50% strength acetic acid and 53 g of 10%strength NaOH formed the initial charge in a polymerization vessel of 15liters volume with a stirrer and inlets for four feeds, and the mixturewas heated to 60° C. The rotation rate was 180 rpm. Once the reactionconditions had been established, addition of 630 g of 20% strengthaqueous sodium persulfate solution and 630 g of 10% strength aqueousascorbic acid solution was begun, in each case at 245 g/h. After 5minutes the monomer feed was begun, composed of 2610 g of 50% strengthaqueous AMPS solution, 7260 g of 18% strength aqueous methacrylamidesolution, 53 g of styrene and 200 g of 10% strength NaOH, at 5060 g/h.The regulator feed, composed of 133 g of acetone and 40 g of laurylmercaptan, was begun at the same time, at 86.6 g/h. Both feeds ran for120 minutes. The internal temperature was regulated so as to maintain areaction temperature of 60° C. After the monomer feed had ended, feed ofinitiator continued for a further 30 minutes. The acetone was distilledoff by applying slightly reduced pressure, and the mixture was thencooled and discharged.

This gave an aqueous polymer with a polymer content of 21.5%, aviscosity of 40 mPas (Brookfield BF20, 23° C.) and a pH of 4.5. The Kvalue was 40.

EXAMPLE 2

334 g of water, 5.5 g of 50% strength acetic acid and 7.3 g of 10%strength NaOH formed the initial charge in a polymerization vessel of 3liters volume with a stirrer and inlets for four feeds, and the mixturewas heated to 60° C. The rotation rate was 180 rpm. Once the reactionconditions had been established, addition of 87 g of 20% strengthaqueous sodium persulfate solution and 87 g of 10% strength aqueousascorbic acid solution was begun, in each case at 55 g/h. After 5minutes the monomer feed was begun, composed of 365 g of 50% strengthaqueous AMPS solution, 1010 g of 18% strength aqueous methacrylamidesolution, and 27.3 g of 10% strength NaOH, at 1400 g/h. The regulatorfeed, composed of 18.2 g of acetone and 5.5 g of lauryl mercaptan, wasbegun at the same time, at 23.7 g/h. Both feeds ran for 60 minutes. Theinternal temperature was regulated so as to maintain a reactiontemperature of 60° C. After the monomer feed had ended, feed ofinitiator continued for a further 30 minutes. The acetone was distilledoff by applying slightly reduced pressure, and the mixture was thencooled and discharged.

This gave an aqueous polymer with a polymer content of 21.6%, aviscosity of 25 mpas (Brookfield BF20, 23° C.) and a pH of 3.8. The Kvalue was 28.

EXAMPLE 3

545 g of water, 5.5 g of 50% strength acetic acid and 7.3 g of 10%strength NaOH formed the initial charge in a polymerization vessel of 3liters volume with a stirrer and inlets for four feeds, and the mixturewas heated to 0° C. The rotation rate was 300 rpm. Once the reactionconditions had been established, addition of 87 g of 20% strengthaqueous sodium persulfate solution and 87 g of 10% strength aqueousascorbic acid solution was begun, in each case at 34 g/h. After 5minutes the monomer feed was begun, composed of 295 g of 50% strengthaqueous AMPS solution, 818 g of 18% strength aqueous methacrylamidesolution, 73.7 g of methacrylic acid and 27.3 g of 10% strength NaOH, at607 g/h. The regulator feed, composed of 18.4 g of acetone and 5.5 g oflauryl mercaptan, was begun at the same time, at 12 g/h. Both feeds ranfor 60 minutes. The internal temperature was regulated so as to maintaina reaction temperature of 60° C. After the monomer feed had ended, feedof initiator continued for a further 30 minutes. The acetone wasdistilled off by applying slightly reduced pressure, and the mixture wasthen cooled and discharged.

This gave an aqueous, polymer with a polymer content of 21.9%, aviscosity of 27 mPas (Brookfield BF20, 23° C.) and a pH of 4.5. The Kvalue was 26.

Preparation of the Protective-colloid-stabilized Dispersions

EXAMPLE 4

4600 g of water, 2000 g of the protective colloid from Example 2, andalso 3650 g of vinyl acetate formed the initial charge in an autoclaveof 15 liters volume with a stirrer and inlets for three feeds, and alsoan ethylene supply, and the mixture was heated to 65° C. The rotationrate was 300 rpm. 64 bar of ethylene pressure were then applied and theethylene supply was shut off. Once the reaction conditions had beenestablished, 380 g of 3% strength aqueous potassium persulfate solutionand 380 g of 1.5% strength aqueous Brüggolit solution were fed, in eachcase at 60 g/h. 45 minutes after the start the initiator feed wasincreased to 90 g/h. After 80 minutes, 1620 g of vinyl acetate were fedat 700 g/h. The internal temperature was regulated so as to maintain areaction temperature of 65° C. The pressure was held at 75 bar until thedesired amount of ethylene was reached. Once monomer feed had ended, theinitiator feed continued for a further hour. The mixture was then cooledand, from 60° C. released with pressure reduction into a suitablereactor, as a result of which excess ethylene was disposed of into theexhaust-gas system. To reduce residual monomers, 40 g of tert-butylhydroperoxide (10% strength) and 40 g of Brüggolit (10% strength) wereadded and the mixture was stirred for 1 hour. 15 g of Hydorol W(commercially available product from Biochema Schwaben) were then addedto the dispersion as preservative, and it was discharged via a 0.5 μmscreen.

This gave a dispersion with a polymer content of 50.3%, a viscosity of50 mPas (Brookfield BF20, 23° C.) and a pH of 3.0. The dynamic glasstransition temperature was −14° C. Determination of particle sizedistribution by Coulter LS230 gave a Dw of 830 nm.

EXAMPLE 5

1500 g of water, 653 g of a 21.5% strength aqueous solution of theprotective colloid from Example 1, and also 1190 g of vinyl acetateformed the initial charge in an autoclave of 5 liters volume with astirrer and inlets for three feeds, and also an ethylene supply, and themixture was heated to 65° C. The rotation rate was 300 rpm. 64 bar ofethylene pressure were then applied and the ethylene supply was shutoff. Once the reaction conditions had been established, 150 g of 3%strength aqueous potassium persulfate solution and 150 g of 1.5%strength aqueous Brüggolit solution were fed, in each case at 30 g/h.After 60 minutes, 530 g,of vinyl acetate were fed at 264 g/h. Theinternal temperature was regulated so as to maintain a reactiontemperature of 65° C. The pressure was held at 75 bar until the desiredamount of ethylene was reached. Once monomer feed had ended, theinitiator feed continued for a further hour. The mixture was then cooledand, from 60° C., released with pressure reduction into a suitablereactor, as a result of which excess ethylene was disposed of into theexhaust-gas system. To reduce residual monomers, 13 g of tert-butylhydroperoxide (10% strength) and 13 g of Brüggolit (10% strength) wereadded and the mixture was stirred for 1 hour. 3 g of Hydorol W(commercially available product from Biochema Schwaben) were then addedto the dispersion as preservative, and it was discharged via a 0.5 μmscreen.

This gave a dispersion with a polymer content of 51.3%, a viscosity of70 mPas (Brookfield BF20, 23° C.) and a pH of 3.1. The dynamic glasstransition temperature was −9° C. Determination of particle sizedistribution by Coulter LS230 gave a Dw of 850 nm.

EXAMPLE 6

405 g of water, 411 g of 21% strength aqueous solution of the protectivecolloid from Example 2, and also 24 g of styrene and 24 g of butylacrylate formed the initial charge in an autoclave of 3 liters volumewith a stirrer and inlets for three feeds, and the mixture was heated to75° C. The rotation rate was 220 rpm. Once the reaction conditions hadbeen established, 155 g of 3% strength aqueous potassium persulfatesolution and 155 g of 1.5% strength aqueous Brüggolit solution were fed,in each case at 50 g/h. After 5 minutes, 455 g of styrene and 455 g ofbutyl acrylate were fed at 303 g/h. The internal temperature wasregulated so as to maintain a reaction temperature of 75° C. Oncemonomer feed had ended, the initiator feed continued for 2 hours. Themixture was then cooled. To reduce residual monomers, 4.2 g oftert-butyl hydroperoxide (10% strength) and 4.2 g of Brüiggolit (10%strength) were added and the mixture was stirred for 1 hour. 3 g ofHydorol W (commercially available product from Biochema Schwaben) werethen added to the dispersion as preservative, and it was discharged viaa 0.5 μm screen.

This gave a dispersion with a polymer content of 43.1%, a viscosity of50 mpas (Brookfield BF20, 23° C.) and a pH of 2.4. The dynamic glasstransition temperature was 19° C. Determination of particle sizedistribution by Coulter LS230 gave a Dw of 340 nm.

EXAMPLE 7

1340 g of water, 705 g of a 21% strength aqueous solution of theprotective colloid from Example 3, and also 1300 g of vinyl acetateformed the initial charge in an autoclave of 5 liters volume with astirrer and inlets for three feeds, and also an ethylene supply, and themixture was heated to 65° C. The rotation rate was 300 rpm. Once thereaction conditions had been established, 245 g of 3%, strength aqueouspotassium persulfate solution and 245 g of 1.5% strength aqueousBrüggolit solution were fed, in each case at 30 g/h. After 60 minutes,575 g of vinyl acetate were fed at 230 g/h. The internal temperature wasregulated so as to maintain a reaction temperature of 65° C. Thepressure was held at 75 bar from 70 minutes after the start of thereaction until monomer feed ended. Once monomer feed had ended, theinitiator feed continued for 2 hours. The mixture was then cooled and,from 60° C., released with pressure reduction into a suitable reactor,as a result of which excess ethylene was disposed of into theexhaust-gas system. To reduce residual monomers, 12 g of tert-butylhydroperoxide (10% strength) and 12 g of Brüggolit (10% strength) wereadded and the mixture was stirred for 1 hour. 3 g of Hydorol W(commercially available product from Biochema Schwaben) were then addedto the dispersion as preservative, and it was discharged via a 0.5 μmscreen.

This gave a dispersion with a polymer content of 50.5%, a viscosity of30 mPas (Brookfield BF20, 23° C.) and a pH of 5.7. The dynamic glasstransition temperature was −8° C. Determination of particle sizedistribution by Coulter LS230 gave a Dw of 920 nm.

To assess the water resistance of the dispersion polymers obtainableusing the novel process, the resistance of the polymer films obtainabletherewith and the resistance of dry mortar compositions modified usingthe dispersion polymers were tested: Water resistance of the polymerfilm:

A dispersion film of wet thickness 500 μm was drawn on a glass plateusing a doctor. The film was then dried at 60° C. for 24 h. To test thewater resistance a pipette was used to place a drop of water on thefilm. After 60 sec the drop of water was rubbed over the film and thewater resistance was assessed visually. The higher the water resistance,the lower the degree of dispersion of the film in water. The waterresistance was evaluated qualitatively using the following gradingsystem:

Grade 1: film does not disperse at all

Grade 2: film disperses only partially

Grade 3: film disperses slowly

Grade 4: film disperses immediately

Water Resistance of the Cement Film

Equal parts by weight of cement and dispersion (solid/solid) were usedto prepare a dry mortar which was converted into a usable composition byadding water. A doctor was used to draw a cement film of 500 μmthickness, and this was dried for 48 h at room temperature. A pipettewas used to place a drop of water on this cement film, and the whiteningof the film and the dispersion when rubbed with the finger were assessedvisually. The water resistance was evaluated qualitatively using thefollowing grading system:

Grade 1: film does not whiten or disperse

Grade 2: film whitens somewhat but does not disperse

Grade 3: film whitens severely and disperses to some extent

Grade 4: film disperses markedly

Grade 5: film disperses completely

For comparison, as Comparative Example 1, a polyvinyl-alcohol-stabilizedvinyl acetate-ethylene copolymer dispersion (ethylene content: 11%) and,as Comparative Example 2, a polyvinyl-alcohol-stabilized vinylacetate-ethylene copolymer dispersion (ethylene content: 21%) were used.The results are given in the table. The dispersions obtained using thenovel process exhibit good water resistance, both in the dispersion filmand in the cement film. In contrast, the comparative examples show thatthe polyvinyl-alcohol-stabilized products are very susceptible to attackby water.

TABLE Water resistance Example Polymer film Cement film Ex. 4 1 1 Ex. 51 1 Ex. 6 1 1 Ex. 7 1 1 Comp. Ex. 1 4 4 Comp. Ex. 2 4 4

We claim:
 1. A process for preparing protective-colloid-stabilizedaqueous polymer dispersions of water insoluble polymers suitable for thepreparation of water-redispersible polymer powders, by free-radicalemulsion polymerization of ethylenically unsaturated monomers, whichcomprises carrying out the polymerization in the presence of one or moreprotective colloids comprising a) from 20 to 95% by weight of monomerunits containing sulfonic acid groups or sulfonate groups, b) from 5 to80% by weight of monomer units containing amide groups or hydroxyalkylgroups, and c) from 0 to 5% by weight of water-insoluble ethylenicallyunsaturated hydrophobic monomer units.
 2. The process as claimed inclaim 1, wherein the polymerization is carried out in the presence ofprotective colloids having a) from 40 to 60% by weight of monomer unitswhich derive from one or more monomers selected from the classconsisting of 2-acrylamido-2-methylpropane-sulfonic acid,styrenesulfonic acid, sulfoalkyl (meth)acrylates, sulfoalkyl itaconates,preferably in each case having a C₁-C₆-alkyl radical, vinylsulfonic acidand salts thereof, and b) from 40 to 60% by weight of monomer unitswhich derive from acrylamide and/or from methacrylamide.
 3. The processas claimed in claim 1, wherein one or more monomers selected from theclass consisting of vinyl esters of unbranched or branched carboxylicacids having from 1 to 18 carbon atoms, esters of acrylic or methacrylicacid with unbranched or branched alcohols having from 1 to 18 carbonatoms, vinylaromatics, vinyl halides and olefins are polymerized.
 4. Abinder composition comprising the protective-colloid-stabilized polymerdispersion prepared by the process of claim
 1. 5. A binder compositioncomprising the protective-colloid-stabilized polymer powder prepared bythe process of claim
 2. 6. An adhesive composition comprising theprotective-colloid-stabilized polymer dispersion prepared by the processof claim
 1. 7. The process of claim 1, wherein said polymer stabilizedby said protective colloid is a polymer prepared by polymerizing vinylacetate monomer and optionally one or more comonomers from the group ofvinyl esters other than vinyl acetate, (meth)acrylates, and α-olefins.8. The process of claim 1 wherein said polymer stabilized by saidprotective colloid is a polyvinyl acetate homopolymer or a polyvinylacetate/ethylene copolymer.
 9. The process of claim 1, wherein saidethylencially unsaturated monomers of the polymer of said aqueouspolymer dispersion are selected from the group consisting of vinylesters of branched or unbranched C₁₋₁₈ carboxylic acids, (meth)acrylicesters of C₁₋₁₈ branched or unbranched alcohols, vinylaromatic monomers,vinyl halide monomers, olefin monomers, and mixtures thereof, andwherein said polymers optionally contain from 0.5 to 5.0 percent byweight of one or more crosslinking monomers selected from the groupconsisting of N-methylolacrylamide, N—(C₁₋₆alkoxymethyl)acrylamides, andN—(C₁₋₆alkoxymethyl)methacrylamides; from 0.5 to 5.0 percent by weightof comonomers with more than one ethylenic unsaturation selected fromthe group consisting of ethylene glycol diacrylate, divinyl adipate,divinylbenzene, diallyl phthalate and triallyl cyanuarate; and/or 0.5 to5.0 weight percent of adhesion-modifying comonomers selected from thegroup consisting of hydroxylalkyl methacrylates, hydroxyalkyl acrylates,diacetoneacrylamide, and acetylacetoxyethyl (meth)acrylate.